The present invention relates to a telephone interface circuit and in particular relates to an improvement in which the telephone interface circuit is disconnected from a telephone line when extension-to-extension calls are made.
FIG. 1 shows the circuit structure of a telephone interface circuit 10 in the prior art. The telephone interface circuit 10 controls the interface between a speech circuit 30 and telephone lines L1 and L2. The telephone interface circuit 10 mainly comprises a varistor V1, a diode bridge 20, a hook switch 40, and transistors Q1 and Q2.
Transistor Q1 is a PNP transistor, and transistor Q2 is an NPN transistor. A resistor R1 is connected from an emitter terminal E1 to a base terminal B1 of the transistor Q1. The emitter terminal E1 of the transistor Q1 is connected to the cathode of a diode D1, and a collector terminal C1 is connected to the speech circuit 30. A resistor R10 is connected between the base terminal B1 of the transistor Q1 and a collector terminal C2 of the transistor Q2. An emitter terminal E2 of the transistor Q2 branches in two directions, in one direction the emitter terminal E2 is connected to the speech circuit 30 and in the other direction the emitter terminal E2 is connected to the anode of a diode D4. A resistor R527 is connected between a base terminal B2 and the emitter terminal E2 of the transistor Q2. The base terminal B2 of the transistor Q2 is connected to a microcomputer (not shown in the drawings) via a resistor R4. The microcomputer (not shown in the drawings) controls the electric potential of a terminal HC at times of on-hook operations, off-hook operations, dial pulse transmitting operations, and the like. Also, the base terminal B2 of the transistor Q2 is connected to a terminal 1 of the hook switch 40 via a resistor R21.
The varistor V1 absorbs voltage surges on the telephone lines L1 and L2 (for example high voltage of the order of 270 volts).
The diode bridge 20 rectifies the signal from telephone lines L1 and L2 and supplies the signal to the speech circuit 30. The diode bridge 20 is formed from four diodes, D1 to D4.
The hook switch 40 connects the terminal 1 and a terminal 3 when it is in an off-hook state. In this state forward voltage is supplied between the base terminal B2 and the emitter terminal E2 of the transistor Q2 via the resistor R21, thus the transistor Q2 turns on. Then the electric potential at the base of the transistor Q1 rises causing transistor Q1 to also turn on. The microcomputer (not shown in the drawings) controls the electric potential of the base of the transistor Q2 via the terminal HC, in response to the input of a dialed number at the time a call is initiated. Thus the transistor Q1 transmits a dial pulse signal.
The hook switch 40 connects the terminal 1 and a terminal 2 when in an on-hook state. Then the transistor Q1 turns off, disconnecting the speech circuit 30 from the telephone lines L1 and L2.